This invention relates to a method using a low frequency active switch for improving input power factor in power supplies utilizing a full wave rectifier and capacitor filter for supplying a filtered DC voltage from an AC power source. More specifically, it would be applied, but not limited to, high frequency power supplies which utilize direct off-line rectification and filtering of an AC source to generate a high voltage DC input to a high frequency DC to DC converter load circuit.
In the conventional rectifier and filter systems the input power factor is typically in the range 0.65 to 0.70. Power factor is defined here as input power (watts) divided by the product if AC input voltage (Volts RMS) and AC input current (Amperes RMS). EQU PF=(P.sub.in /E.sub.RMS I.sub.RMS)
Ideally this factor is unity (1.0) if the load seen by the AC source is purely resistive. In the typical rectifier and capacitor filter circuit the AC input current occurs in pulses, is highly distorted, and has a fundamental frequency component which is phase displaced with respect to the AC input voltage waveform. These factors cause the RMS value of the AC input current to be high and the resultant power factor to be low, i.e. between (0.65 and 0.70).
A conventional technique for improving power factor is to utilize tuned passive filters between the AC source and rectifier circuit. These are comprised of linear inductors and capacitors connected in various series and parallel circuits. These techniques are costly, large in size, frequency sensitive, provide significant loss in efficiency, and cannot provide significant improvement over wide AC input voltage, input frequency, and load ranges.
A second widely used technique is to provide a high frequency switching circuit which switches and controls the AC input current to provide an approximately sinusoidal waveform. These techniques provide a high degree of waveform control and power factor improvement, however they are costly, generate high noise, result in significant efficiency loss, and have reduced reliability.